The invention relates to rotary drill bits for use in drilling or coring deep holes in subsurface formations.
In particular, the invention is applicable to rotary drill bits of the kind comprising a bit body having a shank and an inner channel for supplying drilling fluid to the face of the bit, and where the bit body carries a plurality of so-called "preform" cutting elements. Each cutting element is in the form of a tablet, usually circular, having a hard cutting face formed of polycrystalline diamond or other superhard material.
Conventionally, each cutting element is formed in two layers: a hard facing layer formed of polycrystalline diamond or other superhard material, and a backing layer formed of less hard material, such as cemented tungsten carbide. The two layer arrangement not only permits the use of a thin diamond layer, thus reducing cost, but also provides a degree of self-sharpening since, in use, the less hard backing layer wears away more easily than the harder cutting layer.
In one commonly used method of making rotary drill bits of the above-mentioned type, the bit body is formed by a powder metallurgy process. In this process a hollow mould is first formed, for example from graphite, in the configuration of the bit body or a part thereof. The mould is packed with powdered material, such as tungsten carbid, which is then infiltrated with a metal alloy, such as a copper alloy, in a furnace so as to form a hard matrix.
Where such a method is used to make a drill bit using natural diamond cutting elements; the diamonds are conventionally located on the interior surface of the mould before it is packed with tungsten carbide, so that the diamonds become embedded in the matrix during the formation of the bit body. The maximum furnace temperature required to form the matrix may be of the order of 1050.degree.-1170.degree. C, and natural diamonds can withstand such temperatures. Conventional preforms, however, are only thermally stable up to a temperature of 700.degree.-750.degree. C. For this reason preform cutting elements are normally mounted on the bit body after it has been moulded, and the interior surface of the mould is suitably shaped to provide surfaces to which the cutting elements may be subsequently hard soldered or brazed, or to provide sockets to receive studs or carriers to which the cutting elements are bonded.
This subsequent mounting of the cutting elements on the body is a time-consuming, difficult and costly process due to the nature of the materials involved, and, due to these difficulties, the mounting of some elements on the bit body is sometimes inadequate, giving rise to rapid fracture or detachment of the elements from the drill bit when in use. Furthermore, the mounting methods which have been developed, although generally effective, sometimes for reasons of space, impose limitations on the positioning of the cutting elements on the bit body.
There are, however, now available polycrystalline diamond materials which are thermally stable up to the . infiltration temperature, typically about 1100.degree. C. Such a thermally stable diamond material is supplied by the General Electric Company under the trade name "GEOSET".
This material has been applied to rotary drill bits by setting pieces of the material in the surface of a bit body so as to project partly from the surface, using a similar method to that used for natural diamonds. The pieces have been, for example, in the form of a thick element of triangular shape, one apex of the triangle projecting from the surface of the drill bit and the general plane of the triangle extending either radially or tangentially. However, since such thermally stable elements do not have a backing layer to provide support, they are of substantially greater thickness, in the cutting direction, than conventional preforms in order to provide the necessary strength. This may significantly increase the cost of the cutting elements. Furthermore, the increase in thickness means that the cutting elements are no longer self-sharpening since the portion of the element behind the cutting face does not wear away faster than the cutting face itself, as is the case, as previously mentioned, with two-layer cutting elements.
It is therefore an object of the present invention to provide a rotary drill bit using thermally stable cutting elements, in which the above-mentioned disadvantages of such elements may be overcome. The invention also provides a method of making a rotary drill bit using thermally stable cutting elements.